1. Field of the Invention
The present invention relates generally to holography, and in particular, to an ultrahigh density and ultrahigh bandwidth holographic memory.
2. Description of the Related Art
Many devices (e.g., compact discs and digital video discs) use light to store and read data. However, prior art optical storage methods have limited transfer and capacity capabilities. To overcome the disadvantages of the prior art, holographic memory may be used. Holographic memory stores information beneath the surface of the recording medium and uses the volume of the recording medium for storage. However, holographic memory may have speed limitations with respect to recording data and/or reading the data from the storage medium. In addition, holographic memory density may be limited. These problems may be better understood by describing the future needs for memory and prior art holographic memory systems.
Current technology, as driven by the personal computer and commercial electronics market, is focusing on the development of various incarnations of Static Random Access Memory (SRAM), Dynamic Random Access Memory (DRAM), and Flash memories. Both DRAM and SRAM are volatile. Their densities are approaching 256 Mbits per die. Advanced 3-D multichip module (MCM) packaging technology has been used to develop solid-state recorder (SSR) with storage capacity of up to 100 Gbs. The flash memory, being non-volatile, is rapidly gaining popularity. Densities of flash memory of 256 Mbits per die exist in the prior art. High density SSR could also be developed using the 3-D MCM technology. However, flash memory is presently faced with two insurmountable limitations: limited endurance (breakdown after repeated read/write cycles), and poor radiation-resistance (due to simplification in power circuitry for ultra-high density package).
NASA's future missions may require massive high-speed onboard data storage capability to support Earth Science missions. With regard to Earth science observation, a 1999 joint Jet Propulsion Laboratory and Goddard Space Flight Center (GFSC) study (“The High Data Rate Instrument Study”) has pointed out that the onboard science data (collected by high date rate instruments such as hyperspectral and synthetic aperture radar) stored between downlinks would be up to 40 terabits (Tb) by 2003. However, onboard storage capability in 2003 was estimated at only 4 Tb that is only 10% of the requirement. The storage capability has fallen further behind and now likely supports merely 1% of the onboard storage requirements.
Accordingly, prior art electronic memory cannot satisfy all NASA mission needs. Thus, what is needed is a new memory technology that would simultaneously satisfy non-volatility, rad-hard, long endurance as well as high density, high transfer rate, low power, mass and volume to meet all NASA mission needs.
Volume holography has been predominantly considered as a high-density data storage technology. With volume holography, the volume of the recording medium is utilized for storage instead of only utilizing the surface area (such as with compact discs [CDs] and/or digital video discs [DVDs]). Traditionally, when a laser is fired, a beam splitter is utilized to create two beams. One beam, referred to as the object or signal beam/wavefront travels through a spatial light modulator (SLM) that shows pages of raw binary data as clear and dark boxes. The information from the page of binary code is carried by the signal beam to a light-sensitive lithium-niobate crystal (or any other holographic materials such as a photopolymer in place of the crystal). The second beam (produced by the beam splitter), called the reference beam, proceeds through a separate path to the crystal. When the two beams meet, the interference pattern that is created stores the data carried by the signal beam in a specific area in the crystal as a hologram (also referred to as a holographic grating).
Depending on the angle of the reference beam used to store the data, various pages of data may be stored in the same area of the crystal. To retrieve data stored in the crystal, the reference beam is projected into the crystal at exactly the same angle at which it entered to store that page of data. If the reference beam is not projected at exactly the same angle, the page retrieval may fail. The beam is diffracted by the crystal thereby allowing the recreation of the page that was stored at the particular location. The recreated page may then be projected onto a charge-coupled device (e.g., CCD camera), that may interpret and forward the data to a computer.
Thus, as described above, a complex data-encoded signal wavefront is recorded inside a media as sophisticated holographic gratings by interference with a selective coherent reference beam. The signal wavefront is recovered later by reading out with the same corresponding reference beam.
Bragg's law determines that the diffracted light intensity is significant only when the diffracted light is spatially coherent and constructively in phase. Bragg's law is often used to explain the interference pattern of beams scattered by crystals. Due to the highly spatial and wavelength Bragg selectivity of a crystal, a large number of holograms can be stored and read out selectively in the same volume. Accordingly, there is a potential for one bit per wavelength cube data storage volume density and intrinsic parallelism of data accessing up to Mbytes per hologram.
In addition, it can be noted that prior art holographic memory density can reach 1 terabyte by using binary holograms (i.e., pixel value is either 0 or 1). With such a system, a transfer rate can be achieved with up to 1 Gbit/sec. However, such system may still be at least one order of magnitude lower than many current storage needs.
The storage density and transfer rate using the prior art binary-valued holograms used in such a system cannot be increased by one order of magnitude without significantly increasing the system volume and power consumption. Accordingly, a new holographic memory architecture may be necessary to satisfy the density/bandwidth needs while meeting the volume/mass/power consumption constraints of many applications (e.g., NASA and Department of Defense applications).
Accordingly, as described above, the prior art fails to provide sufficient memory capabilities. Prior art holographic memory systems have evolved in an attempt to provide such capabilities. However, the prior art holographic memory systems may still be improved in storage capacity/density, efficiency, speed, resistance to radiation, etc.